Hi,
Sorry, I am not in the UK, but here's a couple of links that you have probably already found. I'll keep looking.
http://www.livefoods.co.uk/default.php?cPath=26
Found the information on this site on a forum:
Livefood UK will be stocking the product for those of you who like on-line ordering or you can reach the company direct to order over the phone with a credit/debit card - Oxford Silkworms 0793 908 0092
This link seems to be for raise them yourself supplies, but they might know of silkworm farms.
http://www.wwb.co.uk/html/silk.html
You might want to write this company also, they can probably tell you who they supply with the leaves for the silkworms:
http://www.tree-shop.co.uk/products_...headingID=1007
This person may be able to help you, some finches require silkworms:
http://www.robharvey.com